Thermoforming lines are used to manufacture and form a variety of plastic, thin-walled articles by processing a continuous web or sheet of thermoformable plastic material. One particular technique involves the use of continuous web, differential pressure, thermoforming machines which encounter a problem wherein the web of thermoformable material is heated after which the material sags at a heating station before it reaches a molding station. When a thin web of thermoformable plastic material is heated in a heat tunnel, the thin web of material has relatively little “hot strength”. Typically, a thin web of thermoformable plastic material is clamped along its edges as it is conveyed using a thermoforming conveyor through a heat tunnel and into thermoforming machine. However, the strength of the heated, web of plastic material is typically insufficient to fully support the mid portion of the web.
One technique for limiting sagging of a heated web or sheet of thermoformable plastic material entails the use of longitudinally extending, endless sag bands that are configured to support a mid portion of the web, as disclosed in U.S. Pat. No. 2,967,328. However, sag bands are made from spring steel, and they have been known to break. When a sag band breaks, the steel band creates a risk to downstream machinery as the broken band can be fed into a downstream thermoforming press resulting in damage to the press. Additionally, stationary sag bands and wires have also been utilized to support a web of thermoformable plastic material within an oven. However, stationary sag wires have been known to slightly melt into the plastic web of material, leaving blemishes in the surface of the material, which can affect the finished quality of articles formed in the web. Furthermore, the wires have also been known to break, similar to the bands.
Another previously known technique involves the utilization of integral, transversely extending support strips that are formed via a cooling operation in a sheet or web of thermoformable plastic material, as disclosed in U.S. Pat. No. 3,664,791. However, the incorporation of such integral support strips complicates the manufacturing process and can slow it down. Additionally, the strips, which are rigidified, are not completely effective at eliminating sag in all cases.
Even another previous system for inhibiting sagging comprises a sheet support apparatus that utilizes air under pressure within a box that is provided beneath the sheet of thermoformable material in order to float the sheet above the box, as disclosed in U.S. Pat. No. 4,101,252. However, the introduction of a high volume of air against an underside of a web will complicate the uniform thermal heating of the web to a particular desired and uniform temperature. Furthermore, the supply of heated air is interrupted when the web (or sheet) of material is stationary; otherwise, the air might cause chilling of the overlying sheet portion that is being supported thereabove. Such an operation can have a significant negative effect on operating speed because air is compressible and intermittent interruption of the supply of air will take time to support and unsupport the web as the air is supplied and interrupted, respectively.
Accordingly, further improvements are needed to provide a more efficient and effective web support apparatus for delivery of a web of heated thermoformable material through a heat tunnel (or oven) and into a thermoforming machine.